The technology for injecting calcium-based sorbents directly into furnaces to contact high-temperature combustion gases to remove SO.sub.x has employed the use of a variety of reagents, physical means to achieve contact between the pollutants and the reagents, and downstream treatment of the effluent to remove ash and spent reagent. As noted above, the use of wet scrubbers and electrostatic precipitators have been found to be highly effective, but not economically justifiable in many circumstances. Similarly, fluidized beds are effective but costly. It is generally understood that lesser technologies such as in-furnace injection cannot be sufficiently effective to be competitive.
Various "sorbent injection" techniques have been proposed for introducing SO.sub.x sorbents such as lime, limestone and dolomite. These techniques are economically attractive from a capital standpoint in many situations where existing combusters must be cleaned up. However, as currently operated, they need improvement in effectiveness. The idea of sorbent injection is to introduce a suitable sorbent in essentially dry form to react with the SO.sub.x and then collect the spent sorbent by conventional particulate recovery means such as electrostatic precipitation or fabric filters. It has generally been understood that the sorbents should be in chemically-hydrated form, but have been introduced dry, wet, with the fuel, with the combustion air, and at various stages downstream of the primary combustion zone.
One example of dry sorbent injection, is disclosed by Mitchfelder, et al., in U.S. Pat. No. 4,440,100, where they describe introducing the sorbent into a combustor below the burners. Techniques of this type are criticized by Torbov, et al., in U.S. Pat. No. 4,555,996, which indicates that the reaction only occurs within the narrow temperature window of from 1800.degree. to about 2200.degree. F. Torbov, et al. provide data (SO.sub.x reductions of from 22.6 to 72.5%) to show that injecting calcium compounds as slurries is better than injecting the compounds dry form, because the dry form is often rendered inactive when introduced into furnaces at temperatures above about 2200.degree. F. No reaction beyond the indicated temperature zone is identified, and no discussion of particulate recovery is presented.
In European published patent application No. 373,351, DeMichele and Quattroni discuss the use of a mixture of urea, hydrate lime and water in conjunction with an electrostatic precipitator to reduce nitric oxide and sulfur anhydrides. It is indicated that, when the lime concentration is greater than or equal to 35% of the dispersion, a polysaccharide and ligninsulfonate dispersing agent is employed. The key advantage of their process is said to be the improvement in the operation of electrostatic precipitators which is attributed to the use of the urea and lime in aqueous slurry.
The use of slurries is also described by Thompson and Muzio in U.S. Pat. No. 4,731,233. There, the inventors indicate that the use of the slurry with excess water is not the controlling criteria, but it is important to fully hydrate the sorbent prior to introducing it either wet or dry. Their results (SO.sub.x reductions of from 22 to 34%) were obtained on a pilot plant unit and they did not investigate reaction downstream of the primary reaction zone or the advantage of any particular means of particulate collection.
In U.S. Pat. No. 5,058,514, Mozes, Mangal, and Thampi disclose a slurry injection process for SO.sub.2 and NO.sub.x reduction using calcium carbonate and a nitrogenous progenitor such as urea. They reported results of a pilot plant coal burner equipped with an electrostatic precipitator. No interaction was found between the SO.sub.x and NO.sub.x reducing agents. The disclosed advantage is the independent activity of the components--i.e., no loss of activity as compared with independent use of the components. No downstream SO.sub.x reduction was identified.
There is a present need for an economical and efficient process to retrofit existing power plants, especially those burning coal, which provides pollution reductions similar to those achieved using wet scrubbing for SO.sub.x reduction and electrostatic precipitation for particulate removal.